Characteristics of the self absorption of gyrosynchrotron radiation

The characteristics of the self absorption of gyrosynchrotron emission are investigated in detail and its optical depth is estimated for a model of radio microwave source in a magnetic dipole field. It is shown that: (1) the self absorptivity kv∓ decreases rapidly with increasing harmonic number s(= v/vB), so that only the absorption at lower harmonics (s ≤ 5) can affect substantially the microwave burst spectrum; (2) this decrease becomes faster with increasing energy spectral index δ and decreasing low-energy cutoff E0 of energetic electrons; (3) kv∓ increases also obviously with increasing view angle ϑ and reaches maximum around 70°–75°; (4) the optical depth of the self absorption τselfv∓ for 2 ≤ s ≤ 5 varies in the range 101 – 10−2, assuming a number density of energetic electrons of N=103cm−3. This value is 103 – 104 lower than that of the gyroresonance absorption. τselfv∓ would be overestimated in the uniform magnetic field case. Only when N is larger than 104cm−3 does the optical depth of the self absorption become comparable to that of the gyroresonance absorption.